This invention relates to a Faraday rotator which, with little variation in the Faraday rotation angle with temperature, is suited for use in optical isolators, circulators, and switches.
Magnetic garnet materials are known materials constituting Faraday rotators used in optical isolators for the wavelength bands of 1.3 .mu.m and 1.55 .mu.m for optical communications.
Especially, Bi-substituted rare earth-iron garnet materials are promising because of their great Faraday rotation capacities, but they have a disadvantage of rather wide variations in Faraday rotation angle with temperature. Recently, improvements in the temperature characteristics through adjustments of garnet compositions have been proposed.
One example is patent application public disclosure No. 105931/1987. The invention suggests that Tb.sub.2.6 Bi.sub.0.4 Fe.sub.5 O.sub.12 is an excellent composition which reduces the rate of change in rotation angle with temperature to almost zero. Its drawback is a small Faraday rotation coefficient (rotatability) because the Bi substitution is limited so as to improve the temperature characteristic. No material composition has hitherto been known to the art which involves adequately large Bi substitution and is capable of reducing the temperature variation to practically naught.
As an approach other than the adjustment of composition, Utility Model Application Publication No. 9376/1986 discloses a temperature-compensated optical isolator. The amount of temperature variation of Faraday rotation angle is compensated by a change in the applied magnetic field. It is an effective way of improving the temperature characteristic, and the present invention utilizes this principle. The cited invention has shortcomings, however, in that the necessity of magnetic adjusting steel besides a permanent magnet makes the construction complex and that the isolator is intended for use in the wave band of 0.8 .mu.m.
The magnetic garnet materials presently in use as Faraday rotators for near infrared wavelengths (1.3 .mu.m and 1.55 .mu.m) are usually used in saturation magnetic fields. They are superior in that they undergo no change in Faraday rotation angle despite minor changes in the external magnetic field. However, as noted above, the larger the Bi content the lower the temperature characteristic, and the smaller the Bi content the smaller the rotatability and the thicker the film required.
Generally, as typically shown in FIG. 1, the application of a magnetic field H to a rare earth-iron garnet causes a change in the Faraday rotation angle. Above the saturation magnetic field H.sub.s, the Faraday rotation angle at the saturation value .THETA..sub.fs will no longer change.
As a Faraday rotator for optical isolator this garnet is usually used in a saturation magnetic field (H&gt;H.sub.s), and the .THETA..sub.fs often is simply called Faraday rotation angle.
A rare earth-iron garnet with a large Bi substitution is advantageously characterized by a large Faraday rotatability (.THETA..sub.fs /thickness) but, on the other hand, has the disadvantage of substantial changes in the .THETA..sub.fs with temperature. For example, with a Bi.sub.x R.sub.3-x Fe.sub.5 O.sub.12 (where R is a rare earth), usually the temperature coefficient of .THETA..sub.fs is about -0.15%/.degree.C. when x&gt;0.5.
In brief, a Faraday rotator of a composition which involves limited change in rotation angle with temperature as taught in patent application Public Disclosure No. 105931/1987 has a low Faraday rotation coefficient (rotatability) because of limited Bi substitution. On the other hand, the device equipped with a temperature compensator as in Utility Model application Publication No. 9376/1986 is complex in construction and requires cumbersome adjustments.
The present invention therefore has for its object the provision of a Faraday rotator which is low in temperature dependence and large in Faraday rotation angle.